Many fuel cells use hydrogen (H2) as a fuel and oxygen (typically in the form of air) as an oxidant. The hydrogen used in the fuel cell can be produced from the reformation of fuels that include hydrogen (for example, methanol or gasoline). The reforming of fuels that include hydrogen may be accomplished using a variety of techniques including: (1) steam reforming in which the fuel in gaseous form reacts with steam; (2) partial oxidation in which the fuel reacts with oxygen or air in proportions less than that needed for complete oxidation; or (3) autothermal reforming in which the fuel partially reacts with steam and partially reacts with oxygen (or air) in a combination steam reforming and partial oxidation type reactor. Steam reforming is more efficient in terms of the yield of hydrogen than partial oxidation. Steam reforming is endothermic while partial oxidation as exothermic. Autothermal reforming falls somewhere in between steam reforming and partial oxidation both in terms of hydrogen yield and the heat addition/removal required.
The selection of a particular reforming process depends upon the particular operation and factors which include the hydrogen yield required, equipment costs and complexity, and the overall process heat requirements. Regardless of the type of fuel reforming reactor utilized, the reformate exiting the reactor typically includes undesirably high concentrations of carbon monoxide which must be removed to prevent poisoning of the catalyst on the fuel cell's anode. The hydrogen-rich reformate/effluent exiting the fuel reforming reactor typically includes carbon monoxide, in about 3–10 mole percent, that must be reduced to very low concentrations, preferably less than 20 ppm, to avoid poisoning the fuel cell anode catalyst.
It is known that the carbon monoxide level of the reformate/effluent exiting a fuel processing reactor can be reduced utilizing a “water gas shift reaction” (WGS) utilizing the excess steam present in the reformate exiting the fuel reforming reactor or wherein water in the form of steam is added to the reformate/effluent exiting the fuel reforming reactor in the presence of a suitable catalyst. This lowers the carbon monoxide content in the reformate according to the following ideal water gas shift reaction:CO+H2O→CO2+H2 (WGS)
About 0.5 mole percent or more CO still survives the water gas shift reaction. The effluent exiting the water gas shift reactor includes hydrogen, carbon dioxide, water, carbon monoxide, and nitrogen.
The water gas shift reaction is a not enough to reduce the CO content in the reformate to an acceptable level of about 20–200 ppm or less. Therefore, it is necessary to further remove carbon monoxide from the hydrogen-rich reformate stream exiting the water gas shift reactor prior to supplying the hydrogen-rich stream to the fuel cell. It is also known to further reduce the CO of the hydrogen-rich reformate exiting the water gas shift rector using a preferential oxidation (PrOx) reaction conducted in a reactor with a suitable catalyst and at a temperature that promotes the preferential oxidation of the CO with the O2 (air) in the presence of the H2 but without consuming or oxidizing substantial amounts of H2 or without triggering a “reverse water gas shift” (RWGS) reaction. The PrOx and RWGS reactions are as follows:CO+½O2→CO2 (PrOx)CO2+H2→H2O+CO (RWGS).
Preferably, the oxygen provided for the PrOx reaction will be about two times the stoichiometric amount required to react the CO in the reformate. If the amount of oxygen exceeds about two times the stoichiometric amount needed, excessive consumption of hydrogen results. On the other hand, if the amount of oxygen is substantially less than about two times the stoichiometric amount needed, insufficient CO oxidation may occur and there is a greater potential for the reverse water gas shift (RWGS) reaction to occur. Therefore, it is typical for the process to be conducted at about four or more times the stoichiometric amount of oxygen that is theoretically required to react with the CO.
PrOx reactors may be either (1) adiabatic wherein the temperature of the reactor is allowed to rise during oxidation of the CO, or (2) isothermal wherein the temperature of the reactors maintain substantially constant during the oxidation of the CO. The adiabatic PrOx process is sometimes affected via a number of sequential stages, which progressively reduces the amount of CO in stages and requires careful temperature control so that the temperature rise is not so great that the reverse water gas shift reaction occurs thereby undesirably producing more CO.
The fuel reforming process of gasoline or other hydrogen containing fuels typically occurs at high temperatures of about 600–800° C. or above. The one notable exception is methanol which can be reformed at temperatures of about 400° C. The water gas shift reaction is typically carried out at a temperature of about 250–450° C. The PrOx reaction typically occurs at about 100–200° C. Therefore, it is necessary for the fuel reforming reactor, the water gas shift (WGS) reactor, and the PrOx reactor to be heated to temperature sufficient for the system to operate properly. However, during startup, conventional fuel processing requires the system components to be heated in stages. This approach leads to an undesirable lag time for bringing the system online. For example, in conventional fuel cell systems it is typical to use boilers, tube and shell type exchangers, or compact bar and plate type exchangers to produce steam from water. These boilers or exchangers are massive and require a substantial amount of heat input to heat up the equipment components before heat can be transferred to the water to create steam. A substantial amount of lag time is thus associated with the use of these types of steam generating equipment. Furthermore, these heavy boilers or exchangers are a disadvantage in mobile applications such as vehicles which are powered at least in part by a fuel cell system. Because there is no direct contact between the combustion source in the boiler or the fluid in the tube and shell heat exchanger, these devices produce pure steam.
Alternatively, external electric heat sources may be employed to bring the components to proper operating temperatures. This approach requires an external electrical source such as a battery, which is heavy, and draws electricity from the system that is designed to generate electricity through the fuel cell. Furthermore, in conventional fuel processing and fuel cell systems, substantial increases on the fuel cell electrical load demand requires rapid delivery of substantial amounts of hydrogen to the fuel cell to accommodate the increase in electrical demand. A substantial lag time has typically occurred in conventional fuel cell systems attempting to respond to such transient conditions.
Therefore, it is desirable to provide a fuel processing system in a fuel cell system that is capable of rapidly producing substantial amounts of heat and hydrogen to quickly achieve high operating temperatures necessary for startup, and is capable of producing substantial amounts of heat and hydrogen necessary to respond to dramatic increases in electrical load demand on the fuel cell during transient conditions. The present invention provides alternatives to and advantages over the prior art.